In general, simple earphones have been widely used for antennas of wireless devices for receiving high frequency radio waves. Alternatively, conductive lines in wireless device bodies have been used for antennas of wireless devices. However, these antennas are difficult to provide sufficient receiver sensitivity. Accordingly, a loop antenna has been proposed as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-84209. In such an antenna, a loop antenna is attached to a neck strap of a wireless device and an inductance element is connected to the loop antenna in parallel so that the aperture surface of the loop antenna is perpendicular to a human body surface.
However, this loop antenna does not take into consideration of an earphone. Therefore, the antenna has a disadvantage in that a signal line of the antenna is separated from a signal line of the earphone. Accordingly, Japanese Unexamined Utility Model Registration Application Publication No. 6-22331 discloses an earphone antenna in which signal lines to the earphone and an antenna are integrated into a helmet.
The antenna disclosed in, for example, the above-described Japanese Unexamined Patent Application Publication No. 10-84209 does not take into consideration of the integration of an earphone at all, which is a problem.
The known earphone antenna in which a signal line for transmitting an audio signal to an earphone and an antenna are integrated has a disadvantage in that a human body significantly affects a wireless device via the antenna since the earphone is in direct contact with the human body, thereby significantly decreasing the stability of the reception.
This problem also occurs in radio receivers. Additionally, this problem is more noticeable in, for example, portable liquid crystal display TVs (television receivers) that receive television broadcast radio waves having reception frequencies higher than that of the radio broadcast waves. However, effective countermeasures to the problem have not been developed yet.
Additionally, when receiving a television broadcast having a wide frequency range, it is difficult to maintain sufficient receiver sensitivity over the wide frequency range.
That is, so-called portable liquid crystal display TVs (television receivers) need to receive high frequency signals in a very wide frequency band such as 90 to 770 MHz. More specifically, in frequency bands for a television broadcast in Japan, a frequency band of 90 to 108 MHz (1 to 3 CH) and a frequency band of 170 to 222 MHz (4 to 12 CH) are used for the VHF range, and a frequency band of 470 to 770 MHz (13 to 62 CH) is used for the UHF range.
It is difficult to obtain sufficiently high receiver sensitivity over such a wide frequency range. Thus, the occurrence of a frequency range having low receiver sensitivity is inevitable. This is because the antenna length (loop length in the case of a loop antenna) determines the reception frequency range, and the receiver sensitivity of the antenna decreases in frequencies outside the frequency range.
To solve this problem, for example, sensitivity control means has been developed in which a magnetic element is movably disposed in a sensitivity control member to adjust the length of the magnetic element inserted into the sensitivity control member in accordance with the received frequency. However, this method requires a troublesome sensitivity control operation every time the received frequency is changed.
Terrestrial digital broadcasting is scheduled to start in the near future. Broadcast radio waves used for the terrestrial digital broadcasting are only in UHF band. Accordingly, the reception frequency band for digital broadcasting receivers is narrower than that for analog broadcasting receivers. However, a known earphone antenna is not suitable for the digital broadcasting receivers. This is because, as described above, the known earphone antenna does not implement effective countermeasures to eliminate the high-frequency affect on the receiver from a human body via the earphone and earphone antenna.
It is an object of the present invention to solve these problems and to provide an earphone antenna that can eliminate the high-frequency influence on a wireless device from a human body via an earphone, that can ensure the receiver sensitivity required for wide-frequency signals without performing a sensitivity control operation, and that can transmit audio signals from a television receiver to an earphone unit. It is another object of the present invention to provide a wireless device including the earphone antenna.